Historically eddy current methods have been used to detect surface, or near surface, anomalies. When applied to carbon fiber reinforced composite materials, eddy current methods can be used to detect and measure a wide variety of anomalies, including the broken fibers associated with impact damage, misaligned fibers, and incomplete densification in carbon/carbon materials. Because many composite materials have high resistivities, inspection is not limited to the surface.
The depth to which a material can be inspected effectively depends on the dimensions of the anomaly to be detected. The diameter of the probe must be at least twice the depth to which the material is to be inspected, and ideally should be 4.5 times the depth. Sensitivity to small anomalies is inversely proportional to the size of the probe. Since significant anomalous regions in composites tend to be much larger than the small pits and cracks of interest in the inspection of metals, this larger size generally is not a drawback with respect to defect sensitivity.
In the past, ferrite cup core eddy current probes used to measure resistivity have been effective in inspecting carbon fiber reinforced composites such as is taught by U.S. Pat. Nos. 4,924,182 and 4,922,201 issued to Vernon et al. on 8 May 1990 and 1 May 1990 respectively. These ferrite cup cores are effective on planar surfaces.
Size is a factor when the composite material to be inspected has a curved surface. Eddy current probes should conform to the surface of the test material. When the thickness of the material to be inspected is large relative to the radius of curvature of the test material, the large probe that is required, if flat, will not conform to the surface. Commercially available axisymmetric cores can be ground to fit an inside diameter surface; grinding to fit an outside diameter surface is next to impossible. Elongated probes, fabricated by gluing together C-shaped or E-shaped cores, can be more easily made to conform to both outside diameter and inside diameter surfaces.
U.S. Pat. No. 4,719,422 issued to deWalle et al on Jan. 12, 1988 teaches a single purpose eddy current probe that is fabricated on a backing of flexible material, shaped to fit an irregular surface, and then made rigid. This results in a single purpose device which can be used to test a particularly shaped test material. When the radius of curvature varies over the surface to be scanned, such as in a rocket nozzle, a rigid probe cannot be used.
An important characteristic of braided carbon/carbon tubes is the exact alignment of carbon fiber toes. Eddy current methods could be useful if resistivity were measured as an elongated probe was rotated from the 0.degree. direction (long axis of probe parallel to tube axis) to the 90.degree. direction (long axis of probe parallel to circumferential direction). Resistivity values would be expected to correlate with fiber alignment between the 0.degree. and 90.degree. directions. The requirement that all parts of the active surface of the probe be equidistant from the surface of the test material, prevents the use of a single rigid elongated probe; when rotated from the 0.degree. to the 90.degree. position increasingly more of the probe surface would not be in contact with the tube. A set of probes would be required, each probe conforming to the surface at the particular orientation.
Adjustable radius probes have potential application to the inspection of components varying in both thickness and resistivity (aircraft wing skins, for example). This type of probe would allow both the separation of these effects and the inspection of a range of thicknesses using the optimum probe size without the need to physically change probes. Adjustable radius probes are made up of concentric ferrite rims and are difficult to fabricate using commercially available ferrite cores. The techniques of constructing an adjustable radius probe are disclosed in U.S. Pat. No. 5,021,738, issued to Vernon et al. on Jun. 4, 1991.
Conventional rigid core eddy current probes either are inefficate in inspecting curved or irregular surfaces or must be specially fabricated to conform to the irregular surface and are thus limited to the particular shape of the test material and quite expensive as a single application item.